We reintroduce the possibility to mount the app in first position in
a target. The option "self" has been dropped since it is now possible
for a component to have several top level nodes.
We re-add the possibility to validate props when dev mode is active.
No change in the API right Now. The dev mode is activated via the
configure method of App class.
Because of a "break" statement instead of "continue", the check for valid
props was stopping as soon as it met an optional props, which kind of
invalidate the whole system.
closes#717
Géry Debongnie